The present invention relates to constant current arc welding in general, and in particular, to arc welders in which voltage and current in the arc zone are controlled or regulated by adjusting the phase angle of thyristors in the power supply for the welding arc.
A constant potential arc welder has been disclosed in copending patent application Ser. No. 891,986, now abandoned, filed by the same applicant on Mar. 31, 1978, in which thyristors are mounted at the secondary side of the autotransformer and where a saturable reactor is serially connected with the arc for limiting current surges during the start and for smoothing out current rectified by the thyristor during the welding operations.
The present invention applies to constant current arc welding. Electric Arc-Welding Apparatus Standard (EWI-1968) of the National Electrical Manufacturers Association (NEMA) defines the constant current arc welder as follows: "A constant-current arc-welding power supply (arc welder) is one which has characteristically drooping volt-ampere curves producing relatively constant current with limited change in load voltage. This type of supply is conventionally used in connection with manual-stick-electrode or tungsten-inert-gas arc welding".
The present invention also relates to direct current arc welders of the type in which current from a main transformer is rectified by thyristors, and regulation of the voltage and/or current is obtained by feedback control from the welding arc zone through controlled adjustment of the thyristor phase angle. Such a power supply has been described in the aforementioned copending application although in the context of a constant voltage arc welder. Thyristor control and the stabilizing effect of a saturable reactor of finite inductance under all practical current conditions have been used to advantage in the arc welder of the aforementioned copending patent application. The same techniques are used with the arc welder according to the present invention. However, in the latter instance since constant current arc welding is the object, specific measures have been taken for arc initiation, for arc stabilization, as well as for insuring an effective welding operation in the normal mode, e.g., while maintaining through the arc the required energy input.
This problem has been encountered in the past and the prior art has given it only piecemeal consideration, and with limited success.
It is an object of the present invention to provide an arc welder in which thyristor control is performed continuously for insuring an adequate current and voltage throughout the welding process.
Another object of the present invention is to provide automatically and successively voltage and current control in relation to welding process conditions.
Still another object of the present invention is to automatically provide current boost through the arc under predetermined boost characteristic for certain low voltage welding conditions.
A further object of the invention is to initiate bumpless transfer from voltage control to current control when initiating the welding process.
A still further object of the present invention is to provide effective and controlled boost current conditions in a constant current mode.
The invention also aims at providing an arc welder capable of monitoring, regulating and controlling the welding operations through phase angle thyristor adjustment continuously in a voltage mode as well as in a current control mode of operation.